1. Field of the Invention
The present invention relates to improvements in a knock sensor which detects vibrations of a cylinder block of an internal combustion engine by means of a piezoelectric element.
2. Description of the Related Art
Generally, to increase a compression ratio of a gasoline engine effectively improves a thermal efficiency of the engine although an octane number of gasoline, which is measure of the anti-knock properties, limits this improvement of the thermal efficiency. In case of a turbo-charged engine, it is important to effectively operate the engine so as not to generate knocking. Therefore, a knocking control system is employed in such a turbo-charged engine so as to maintain the high compression ratio of the engine as possible. Such a knocking control system is arranged to detect vibrations of an engine block by means of a knock sensor and to delay a spark timing of the engine if the knock sensor detects a knocking of the engine. The knock sensor employed in the knocking control system is generally arranged as shown in FIG. 11.
A knock sensor 1 shown in FIG. 11 comprises a cover portion 2 including a connector portion 2A. A housing 3 is constituted by a disc shaped flange portion 3A and a cylinder portion 3B which extends from a center portion of the flange portion 3A in the direction perpendicular to a surface of the flange portion 3A. An external thread portion 3C is formed at an upper portion of the cylinder portion 3B. An upper groove portion 3D is formed at an upper end portion of the cylinder portion 3B and at a portion located in the vicinity of an upper end of the external thread portion 3C. A lower groove portion 3E including three flange portions is formed at an outer periphery of the flange portion 3A. The upper and lower groove portions 3D and 3E are engaged with the cover portion 2 and function to prevent water from entering into an inside space of the cover member 2.
An insulating tube 2 made of a heat-shrinkable material is set at an outer periphery of the cylinder portion 3B. An end of the insulating tube 4 is in contact with the flange portion 3A, and the other end of the insulating tube 4 extends to a center portion of the cylinder portion 3B and in the vicinity of an upper end of the external thread portion 3C. The insulating tube 4 is fittingly connected with the cylinder portion 3B by heating it after installed on the cylinder portion 3b. The insulating tube 4 functions to insulate the cylinder portion 3B from a lower lead plate 5, an piezoelectric element 6, an upper lead plate 7 and a weight.
The lower lead plate 5 made of conductive metal is formed into a ring-shaped disc having a center hole. The lower lead plate 5 is constituted by a disc portion 5A, an arm portion 5B outwardly projecting from an outer periphery of the disc portion 5A, and a terminal portion 5c welded to the arm portion 5B. The lower lead plate 5 is set to be in contact with a lower surface of the piezoelectric element 6 under the assembled condition.
The ring-shaped piezoelectric element 6 is set around the cylinder portion 3B of the housing 3 so as to be in contact with the lower lead plate 5 set on the upper surface of the flange portion 3A. The piezoelectric element 6 is made of piezoelectric material such as lead titanate. The piezoelectric element 6 is, for example, constituted by d33-element which has a characteristic that the directions of a strain axis, a polarization axis, and a signal axis correspond with respect to the axis of the element 6. That is, the piezoelectric element 6 generates a voltage signal corresponding to the oscillating pressure applied in the axial direction thereof.
The upper lead plate 7 made of conductive metal is formed into a ring-shaped disc having a center hole. The upper lead plate 7 is constituted by a disc portion 7A, an arm portion 7B outwardly projecting from an outer periphery of the disc portion 7A, and a terminal portion 7C welded to the arm portion 7B. The upper lead plate 7 is set to be in contact with an upper surface of the piezoelectric element 6 under the assembled condition.
An insulating sheet 8 made of insulating resin is formed into a ring-shaped disc having a center hole. The insulating sheet 8 is installed to the cylinder portion 3B of the housing 3 to be located on an upper surface of the upper lead plate 7 so as to insulate the upper lead plate 7 from the weight 9. The weight 9 made of metal such as steel is formed into a ring-shape or short cylinder and is installed to the cylinder portion 3B of the housing 3 to be located on the insulating sheet 8. The weight 9 together with a disc spring 10 and a nut 11 function to apply a pre-load to the piezoelectric element 6. The disc spring 10 is formed, for example, into an initially coned disc spring or wave washer. The disc spring 10 is installed around the cylinder portion 3B so as to be located on the weight 9. The nut 11 is screwed with the external thread portion 3C of the housing 3. By screwing the nut 11 with the external thread portion 3C, the lower lead plate 5, the piezoelectric element 5, the upper lead plate 7, the insulating sheet 8, the weight 9 and the disc spring 10 are fixed with the housing 3 so that the pre-load is applied to the piezoelectric element 6 through the disc spring 10 and the weight 9. The nut 11 is tightly screwed to the external thread portion 3C of the cylinder portion 3B so as to fixedly sandwich the lower lead plate 5, the piezoelectric element 6, the upper lead plate 7, the insulating sheet 8, the weight 9 and the disc spring 10 between the nut 11 and the flange portion 3A of the housing 3. A resistor for picking up a voltage has a resistance value 560 k.OMEGA. and is installed between the upper and lower lead plate 5 and 7.
The procedure for producing the knock sensor 1 is executed as follow:
(1) The cylinder portion 3B of the housing is covered with the insulating tube 4.
(2) The lower lead plate 5, the piezoelectric element 6, the upper lead plate 7, the insulating sheet 8, and the weight 9 are, in turn, installed to the cylinder portion 3B of the housing 3.
(3) Upon installing the disc spring 10 on the weight 9, the nut 11 is screwed with the external thread portion 3C of the housing 3.
(4) The assembled elements are covered with the cover portion 2 by means of a resin molding. Simultaneously, the connector portion 2A of the cover portion 2 is integrally formed with the cover portion 2 so that the terminal portions 5C and 7C project at the connector portion 2A.
The knock sensor 1 constructed as mentioned above is installed to a cylinder block of the engine by inserting a bolt to the cylinder portion 3B of the housing 3 to detect knocking vibrations of the engine.
However, the conventional knock sensor 1 yet has a possibility to generate the insulation defective thereof even if the insulating tube 4 and insulating sheet 8 are provided therein.